Process of anodizing



Feb. 26, 1963 E. R. RAMIREZ ETAL 3,079,303

PROCESS OF ANODIZI'NG Filed Oct. .7, 195a 2 Sheets-Sheet 1 I INVENTORS'[R/K FEAR/17714 7Z4, ATTORNEYS Feb. 26, 1963 E. R. RAMIREZ ETAL3,079,308

Filed 001:. '7, 1958 INVENTORS 2 F. m RA 7 8 N v 9 BY 46AM- 76:4ATTORNEY) 3,679,308 PROCESS (FF ANODIZTNG Ernest R. Ramirez, Royal Gait,Mich, and Erik F. Barkman, Hem-ice Qonnty, Va, assignors to ReynoldsMetals tCompany, Richmond, Va., a corporation of Deiaware Filed st. 7,$58, Ser. No. 765,844 11 Claims. (Ci. 2'042S) This invention relates toan improved method for anodizing metals in strip form. Moreparticularly, the invention concerns a novel method for the continuoushigh speed anodizing of aluminum articles such as strip, foil, and wire.

This application is a continuation-in-part of our application Serial No.688,670, filed October 7, 1957, and now abandoned.

Presently employed methods of anodizing metals such as aluminum,magnesium, or copper, are based primarily upon batch concepts. In thecase of anodizing aluminum, for example, the operation is ordinarilycarried out employing current densities of the range of 12 to 15 amperesper square foot, for anodizing times varying from 15 to 60 minutes.While the anodic coatings obtainable by this method are adequate inrespect to dielectric properties, ability to absorb dyes, and forcorrosion protection of the underlying metal, batch methods fail to meetthe needs of the times for a rapid, dependable method suitable to massproduction techniques. Efforts have been made in the industry to achievecontinuous anodizing processes. These attempts have heretofore beenunsuccessful, particularly so in the case of aluminum, in that burningor bare spots and nonuniform anodizing were invariably found on coatingmade by continuous anodizing. This was especially true where highcurrent densities were employed.

Experience has shown that the coating on anodized metals consists of arelatively thin, dense, and dielectrically compact barrier layer ofoxide, surmounted by relatively porous outer oxide layer. Theeffectiveness of the overall coating depends to a considerable extentupon the characteristics of the barrier layer, which in turn are relatedto the manner in which the barrier layer is formed.

In accodance with this invention, we have found that by close control ofanodizing conditions during the first few seconds of the anodizingprocess, the character of the barrier oxide layer may be established sothat this layer will exhibit optimum properties. We have found further,in accordance with this invention, that control of the initial anodizingconditions governing the formation of the barrier layer produces auniform, dense and electrically-resistive oxide layer which makes itpossible to carry on subsequent anodization at greatly increased ratesof operation, up to almost any practical desired value. These greatlyincreased rates necessitate resorting to the use of high speed anodizinglines employing multiple cathodes, with regulation of voltage and/ orcurrent density at intervals corresponding to successive cathode sets.It has been further found, in accordance with this invention, thatanodizing rates along the entire length of the metal being anodized maybe regulated and controlled, with a simultaneous substantial saving inpower consumption, by the use of multiple anodizing power sourcesarranged in parallel, these multiple power sources being supplied from amonocyclic square type of power source.

The various aspects of the present invention will be illustrated withrespect to the aanodizing of aluminum strip, foil and wire, but it is tobe understood that the principles as developed and disclosed herein areapplicable with minor variations to the anodizing of other metals suchas magnesium and copper, which are amenable to this treatment.

We have found, in accordance with this invention, that in order toachieve successful results in subsequent anodizing, the initialformation of the barrier layer of aluminum oxide must take place withina period not exceeding about 60 seconds, preferably from about 2 to 6seconds. We have found that by employing carefully balanced control ofimpressed voltage or of current density or both during the first fewseconds, for example, the first four seconds that the aluminum entersthe anodizing liquid, the barrier layer will be uniformly and completelyformed. It requires roughly from 2 to 4 seconds for a good barrier layerto be thus formed. Careful control of the growth of the barrier layerpermits proper control of the subsequent growth and uniformity of theoxide layer subsequently applied in the continuous anodizing operation.The barrier layer preferably has a thickness of between about 0.5% and2% of the thickness of the outer layer.

We have found that formation of the barrier layer in this mannerfacilitates the subsequent formation of the main body of porous oxideusing a gradual build-up of current density which is controlled, inaccordance with the invention, by the increased voltage for the severalmultiple electrode pairs located along the anodizing tank, as will bedescribed below. Immediately following the formation of the initialbarrier layer, pore formation takes place through this layer whichpropagates the formation of addi .tional oxide. Directly between theinitial barrier layer and the metal surface, a new barrier layer isformed, the thickness of which depends on the applied voltage and is ofthe order of 0.5% to 2% of the thickness of the main body of the oxide.In the completed film, the thin dense oxide layer most adjacent to themetal surface becomes the inner layer, and the other part of the oxidebecomes the outer layer.

The key factor, from the industrial standpoint, in continuous anodizing,is the ability to employ high and uniform current densities on thealuminum or other metal strip. Unless the aluminum surface is preparedto take on these high current densities, selective anodizing (burning)will take place on the aluminum surface, thereby producing a nonuniformand inferior type of anodic coating. During conventional anodizing insulfuric acid, at current densities of about 15 to 25 amperes per squarefoot, the formation of the barrier layer is not intentionallycontrolled, but is dependent upon the applied voltage used foraparticular metal or alloy and upon electrolyte conditions.

For example, upon initially immersing the aluminum strip into theanodizing electrolyte, the dense but thin air-formed oxide film on themetal, which is generally in the range of 10 to 50 Angstrom units thick,must be uniformly and rapidly converted into a comparatively thickbarrier layer between about 210 and 280 Angstrom units thick. We havefound that it is during this transformation period, which has been foundto extend from 2 to 4 seconds in duration, that the anodizing processmust be carefully controlled. A gradual and uniform build-up of thiscompact barrier layer during these initial 4 seconds is essential ifhigh current densities are to be subsequently used to produce highquality anodic coatings. Once the barrier layer is uniformly built up toa suitable thickness, then exceptionally high current densities can bereadily and advantageously employed to complete the anodization.Inasmuch as the process of this invention, and especially that phasewhich comprises anodizing in the main part of the system, employsexceedingly high current densities, it is of highest importance thatthere be controlled voltage and current conditions duringthe initialperiod to insure a rapid and complete build-up of the resistivenonporous barrier layer, to prevent burning at isolated spots on thestrip or film. This control is achieved by maintaining relatively closelimits for either or both the voltage and current density in the cell.

Thus, for example, in accordance with this invention, an average currentdensity of about 20 to about 100' amperes per square foot, maintainedwith an average anodizing voltage of about 14 volts for a period ofabout 4 to 6 seconds provides the nearly complete formation of thebarrier layer so as to facilitate subsequent formation of the porouslayer at current densities exceeding 250 amperes per square foot.

In carrying out this initial treatment, in accordance with one mode ofoperation, we have found that control of the quality and uniformity ofthe barrier layer entails the use of higher average voltages, in theneighborhood of 14 to 15 volts, as threshold voltages for the barrierlayer forming step. At the same time the current density is carefullycontrolled, by maintaining it at an initial value of about 80 to 600amperes per sq. ft. when the 14-15 volt average potential is firstapplied, and then reducing the average current density to about 10amperes per sq. ft., during period of a few seconds as the barrier layeris formed. Lower voltages unduly prolong the layer formation time, whilehigher voltages have been found to induce burns, i.e. spots where theelectrical resistance of the initially formed barrier layer isnonuniform, with the result that in the subsequent propagation of theoxide growth with a high current density, a white spot may appear on theanodized surface, indicative of varying thickness ofthe outer layer.

In accordance with our novel method, the anodizing rate is controlledduring the first several seconds after initiating the operation, byeither voltage control or current control. The operation is performedpreferably in a separate anodizing compartment, which may be enclosed,and which will be described more fully below.

The initial anodizing step leading to controlled formation of thebarrier layer may be carried out in any suitable anodizing electrolyteof conventional type, such as an aqueous solution of sulfuric acid,oxalic acid, sulfamic acid, or chromic acid, or combinations thereof.When using sulfuric acid, for example, the concentration of the solutionmay range from 5 to 70 percent in strength, preferably about 30% isused. For the other acids, the concentrations will range between 2% andfor example 5% in the case of oxalic acid. Anodizing bath temperaturesmay range from about 65 to 80 F. in the case of sulfuric acid, and about85 F. in the case of oxalic acid.

According to a second, and preferred mode of operation of our novelprocess, during the first period of not less than about 4 to 6 seconds,when the purpose is primarily to form a barrier layer of oxide, theoperation is controlled at an average current density of the order of 20to 100 amperes per sq. ft., preferably about 50 amperes per sq. ft.Immediately thereafter, the current density may decrease somewhatbecause of the added resistance of the deposited barrier layer. Theimpressed voltage is permitted to increase, in order to maintain theaforesaid level of current density, during which time the barrier layerattains somewhat more than half (about 90 to 110 A. units) of its finalthickness. The initial barrier layer may be built up completely duringthis initial step if the time is extended, but the formation may also,and preferably, be completed during the time When the material beinganodized first is subjected to the main anodizing operation, and at muchhigher current densities, up to 500 or 1000 amperes per sq. ft., butpreferably not in excess of 1000.

While the formation of the barrier layer of oxide may be carried out aseither a batch or continuous operation, the latter will be describedhere, since it forms a part of the continuous anodizing high speedmethod which constitutes the present invention. Preferably directcurrent is employed.

We have found that the barrier layer formation is best carried out bythe use of a battle section which forms a part of the main anodizingtank (see FIG. 1). The name arises from the presence of a baille wall,described more fully in connection with the explanation of theaccompanying drawings, said wall containing a slot to permit passage ofthe metal strip to the main tank. Said slot also serves to permit flowof current via the eletcrolyte to facilitate control of the anodizingrate in the bafile section. Thus, the design of the bafiie section andanodizing conditions in the first cathode section of the main anodizingtank permit close control of the barrier layer formation. The bafilesection has no cathodes of its own. Anodizing in this section iscontrolled by traveling the current through a strip opening and thenthrough relatively lengthy resistance paths. In this way, and by keepingthe average voltage below about 14 to 15 volts, the baffle sectionserves to eliminate possible surges or rapid rises of current which mayset in after about the first 2 seconds of anodizing.

Within the baffle section, which may, for example, be one or two feet inlength, and accommodate, by appropriate weaving, 4 or 5 feet of strip,the controlled initial anodizing of the first 4 seconds, or of the first4 to 6 seconds of the anodizing cycle may be performed so as to becoordinated with the anodizing conditions maintained in the mainanodizing tank, for example, by controlling the size of the slotopening. There may be provided, if desirable, circulation of theelectrolyte within the bafile section, by means, for example, of a pumpsystem, directing a stream of electrolyte against the metal strip, aswell as separate means for adjustment of voltage and current density.

Conducting the barrier layer formation step in a separate baffle sectionin the manner outlined above provides a uniform and continuous barrierlayer, a surface which can be subsequently anodized in the continuousanodizing portion of the system at current densities averaging to 400amperes per sq. ft. Moreover, the speed of passage of the metal stripthrough both the baffle section and the anodizing section may beincreased to from 10 to 100 feet per minute, depending upon the lengthof the system. By the process of this invention, it has been possible toanodize an aluminum strip completely in as little as 24 seconds, and yetobtain very high quality coatings. It is further possible to anodizesuccessfully aluminum strip of thin gauge, for example, from 0.004" to0.008". The anodized surfaces of aluminum strip thus treated exhibitsD.C. breakdown voltages of 470 to 485 volts.

Prior to being led into the baffle section, the aluminum strip may besubjected to a pretreatment with sulfuric acid at a temperature somewhathigher than the anodizing bath temperature in a contact cell (see FIG.1). This treatment serves to condition the aluminum or other metal stripand to form a liquid contact between the source of potential and themetal to be anodized. In such a contact cell, for example, there may becirculated by pump means a 30% sulfuric acid solution at a temperatureof about 100 to F.

In the main anodizing portion of the system, the objective is to achievehigh quality and uniform coatings, by applying the aforesaid outer layerover the initially formed barrier layer. This is accomplished bymaintaining a substantially uniform anodizing rate throughout the entirelength of the anodizing tank. In accordance with this invention bothcontrol and regulation of the anodizing rate along the strip beinganodized are achieved by use of a multicathode arrangement, inconjunction with either (a) a series of variable voltage power sources,or (b) a series of constant current power sources. The latter involvesin addition the use of the monocyclic square arrangement as a constantcurrent power source.

For a better understanding of the invention and its various objects,advantages and details, reference is now made to the present preferredembodiment of the invention which is shown, for purposes of illustrationonly, in the accompanying drawings.

In the drawings:

FIG. 1 is a perspective view showing a complete continuous high speedanodizing system;

FIG. 2 is a perspective view of the baffle section employed to carry outthe formation of the barrier layer.

Referring to FIG. 1, there is a dipicted a complete high speedcontinuous anodizing system comprising a contact cell 2, for immersingthe metal 8, such as aluminum strip, in the electrolyte solution, andsupplying current thereto, a baflie section 4 in which the formation ofthe barrier layer is performed, and a long main anodizing tank orsection 6, in which the main anodizing operation is completed. Contactcell 2 is provided with guide rollers and 12 over which metal strip 8passes and reverses its direction. Current is furnished to the contactcell by means of a lead or graphite anode which is connected through anammeter 15 to the positive terminal of a power source such as a motorgenerator set 16. Pump 18 serves to circulate electrolyte in the contactcell against the metal strip at point 20, thereby cooling the strip,while pump 22 circulates electrolyte in the baflile section 4 againstthe metal strip at point 24. Baffle section 4 is provided with a guideroller 26 around which the metal strip passes immersed in theelectrolyte through the opposite wall of the section, which serves as abattle to separate this portion from the main anodizing section 6, bymeans of adjustable slot 36. If desired, of course, the battle sectioncould be built as a separate unit with guide means provided for passingthe metal strip to the separate main anodizing tank. The main anodizingsection 6 comprises a long tank in which the metal strip passes immersedin the anodizing solution, between consecutively placed pairs ofcathodes, 32-34, 36-38, 49-42, 44-46, which may be for example, leadplates, thence over guide roller 48 and out of the system. Thesecathodes are generally progressively larger in area toward the end ofthe tank. The cathodes are suspended horizontally above and below themetal strip, as shown in FIG. 2, and the respective pairs or sets areinterconnected. The first cathode pair 32-34 is connected through anammeter 50 and a variable resistor 52 to the negative terminal of themotor generator set 16. Subsequent cathode pairs 36-38, 40-42, 44-46,etc. are connected in the same manner to the negative terminal of themotor generator set through ammeters 54, 56, and 58, and variableresistors 60 and 62. The last cathode pair, e.g. 44-46 is connected tothe motor generator set through ammeter 56 without a resistor, since atthis stage of the operation, the resistance of the oxide coating issufficiently high to require no outside resistor. Where the monocyclictype power source is to be employed, e.g. to maintain constant currentsettings through the various cathode pairs by means of variableresistances, the appropriate circuit may be connected across theterminals of the motor generator set.

The temperature of the anodizing solution in the main anodizing tank maybe maintained at any desired level by continuous circulating ofelectrolyte through a cooling apparatus 64 which circulates the solutionto and from various portions of anodizing tank, through circulatingpipes as indicated generally at 66, 68, and 70.

The functions and operation of the various parts of the system willreadily be understood in the light of the previous discussion and of thefollowing observations. The electrolytic contact cell serves tointroduce the unusually high current into the metal strip, and entirelyeliminates the problem of arcing where the current enters the strip. Thestrip leaving the contact cell enters the baffle section of theanodizing system where it is the anode and encounters, for example, asulfuric acid solution at a temperature around 72-75 F. for a period ofpassage of generally less than 6 seconds. The anodizing operationsdescribed herein utilize direct current, but under certaincircumstances, alternating current may be employed with equal facility.

The metal strip then passes through the slot portion of the apparatusinto the main anodizing section which preferably contains the sameelectrolyte as the baffie section. The principle involved here is basedupon a multi-cathode arrangement with careful control of either theanodizing current or the anodizing voltage being carried out by means ofa series of voltage sources connected in parallel, resulting in controlof the anodizing rate along the rapidly moving strip. The arrangement issuch that the potential during anodizing between the moving metal stripand a given cathode pair is controlled at will and is independent of thevoltage relationship existing at any other point in the anodizing tank.This results in case of control and reduced power input. it is superior,for example, to an arrangement using a single power source and aparallel arrangement of resistors to distribute the current evenlythrough a series of spaced cathode pairs. The constant hazard of burningthe metal being anodized as it enters the anodizing tank is totallyeliminated. The method of the present invention uses a parallel seriesof power sources connected between a common anode and a series ofcathode pairs distributed along the anodizing tank, as shown in FIG. 1.

The use of cathode pairs (split cathodes) is preferable to the use ofsingle cathodes spaced along the anodizing tank in that it eliminatesformation of burns by providing controlled rise of current density ateach cathode point. The use of cathode pairs alone, as known in someprocesses, will not, however, alone assure satisfactory results unlessaccompanied by unduly increased voltages or by specific voltage settingsfor each cathode point, and even this would not be enough to assure goodresults unless uniformity of current density can be maintained, i.e. avariation not in excess of about 30%, in passing from one section of thestrip to the next. Moreover, experience has demonstrated that in theanodizing operation there should be a voltage drop of approximately 1volt between adjacent cathode pairs.

In accordance with the present invention any variation in currentdensity is kept below 30%, and the voltage drop is held to about 1 voltbetween adjacent cathode pairs by using circuiting of the monocyclicsquare type. A dense compact film of oxide of superior dielectricproperties is obtained thereby. Using these circuits makes possiblemaintenance of fine control of the current density and voltageconditions within the desired limits.

The monocyclic square power source may be characterized as anarrangement whereby a constant DC. or A.C. current can be maintained byconversion of a constant voltage fed into the circuit, as from a motorgenerator set. The circuit is based upon the principle of a resonantnetwork of reactors and capacitors or capacitor banks connected in aclosed circuit. This resonant network provides a constant current whichis unaffected by the resistance or impedance of the load. Thus themonocyclic square circuit may be employed to secure constant currentsettings through the various cathode pairs of an anodizing tank by meansof variable resistances. Ordinarily," such resistances could absorb asmuch as 10-25% of the power input. This power loss is eliminated by theincorporation of the monocyclic square arrangement into the anodizingcurrent supply.

In anodizing, due to the IR drop along the metal strip, there is adiminution of anodic current density as we go from the beginning of theanodizing strip to the end, when a single cathode arrangement is used.This elfect can be greatly reduced or even reversed by the employment ofmultiple cathodes. The number of cathodes needed to maintain the sameanodizing potential along the entire strip, and the length of thesecathodes is limited by practical factors, such as the cross-sectionalcurrent density in the metal strip, the maximum variation in currentdensity permissible along the strip, and the rate of change of anodizingcurrent density with respect to change in anodizing voltage. The greaterthe permissible variation in current densities along the strip,expressed in percent, the fewer and longer will be the cathodes used. Asmentioned previously, the optimum permissible variation in currentdensity is of the order of 30%.

The ettect of changes in the aforementioned optimum anodizing operatingconditions is shown in the following Table 1:

Hence, in the case of thin gauge material, permitting the averageanodizing current density to rise to 75 amps. per sq. ft., whichcorresponds to a potential of 1.6 volts, which is greater than theoptimum difference of 1 volt, results in burns. This optimum voltagechanges with increasing thickness of the material being anodized.

The effect of variation of current density on the quality of the anodiccoating is further shown in Table 2, below. This table gives summarydata derived from a run using aluminum foil 0.008" thick and /2 wide, inan anodizing tank designed for a maximum. fluctuation of current densitybetween adjacent cathodes of 30%. The tank was 4-8 feet long, and had atotal cathode length of 45 feet. Strip speed was 30 ft. per minute. Thecross-sectional current density was 55,000 amperes per sq. in., the areaof aluminum strip in the anodizing tank was 3.8 sq. ft. Lead strips 2"in width were used as cathodes, and placed at a distance of 5 above andbelow the metal strip. Nine pairs of cathodes of varying length wereemployed, the arrangement of which, and corresponding current densities,were a follows:

In Table 2, the first column refers to sectional variations in anodizingrates within the area covered by a pair of cathodes. The second columnrefers to percentage variation in current density on the strip betweenadjacent pairs of cathodes, the first three percentages being maintainedwithin the optimum limit of about 30%. The fourth column shows effect onthe anodic coating:

TABLE 2 Percent Voltage Sectional Variation in Variation Drop PerQuality of Anodic Anodizing Rates in Current Cathode at Coating Density50,000

Amps/sq. in.

55-50 amp.lsq.ft =F5 0.2 Uniform. -50 amp.lsq.tt. =Fl5 1.0 Uniform.120-50 amp./sq.tt =F30 2.2 Some Burns. 200-50 amp.lsq.it =F40 3.3Frequent Burning.

As mentioned previously, the mode of operation in the baffie sectionwhere the barrier layer of oxide is first formed may involve eitherconstant voltage or constant current control. In the case of constantcurrent control, the current is controlled by means of the monocyclicsquare type of circuit described above. In either case, controlledanodizing during the first 4 to 6 seconds of the cycle is relied upon toproduce the desired barrier layer. This layer provides a surface whichcan subsequently be anodized at high current densities between about 100and 1000 amperes per sq. ft, but preferably between about 100 and 400amperes per sq. ft.

By the use of the novel anodizing process of this invention, stripspeeds of as much as to feet per minute may be attained.

The method of pretreatment and anodizing may be illustrated by thefollowing examples, but the invention is not to be regarded as limitedthereto.

EXAMPLE 1.--CONSTANT VOLTAGE CONTROL Time of the aluminum strip inbaffle section 4 seconds and voltage con- Density in aluminum striptrolwas applied (14 Gauge and width of alumivolts).

num strip 90,000 amps/m Speed of strip .008, V2 inch.

25 feet/minute. 470 volts D.C. breakdown.

Dielectric properties of anodized surface Average anodizing rate in tank220 amps/ft? The anodizing tank has ten cathode pairs. The current drawnby each cathode pair as well as their respective lengths is given inTable 3.

TABLE 3 Conditions Prevailing in 22 Foot Anodizing Tank Average CurrentDensity in Strip Adjacent to Each Cathode Pair, amps/ft.

Average Cathode Sectional Length (feet) Cathode Pair Voltage, Volts D.C.

1 Voltage in the battle section is maintained at 14 volts.

EXAMPLE 2.CONSTANT CURRENT CONTROL Anodizing tank 7 feet long.Electrolyte employed 30% sulfuric acid. Anodizing temperature 76 F.

Anodizing time .4 minute.

Battle section Time of the aluminum strip in bafile section 6 secondsDensity in aluminum strip Gauge and width of aluminum strip Speed ofstrip Dielectric properties of ano- 1 /2 feet long.

applying constant current control (30 amps/fe 30,000 amps/m .008, 2inches.

dizing surface 15 feet/minute. Average anodizing rate in 485 volts DCbreakdown.

tank 250 amps/ft? The 7 foot anodizing tank has 4 ca rode pairs. Table 4shows the conditions prevailing during this test.

TABLE 4 Conditions Prevailing in 7 Foot Anodizing Tank 1 amps/it EXAMPLE3.PREFERRED METHOD Two typical runs made in an apparatus comprising atank 18 feet long having a 2 foot baffle section, and a 16 footanodizing section connected therewith by a slot opening, are described.The electrolyte was 30% sulfuric acid maintained at temperature of 6770F. Aluminum strip of 0.02 inch gauge and 2.25 inches wide was runthrough the apparatus at a speed of 28.5 feet per minute. Direct currentwas employed. The strip was first pre pared by subjecting it to acleaning with a detergent solution, followed by a water rinse, thenfollowed by a hot bright dip bath containing a mixture of nitric andphosphoric acids, followed by another water rinse. Thence the strip waspassed into a contact cell containing 30% sulfuric acid at a temperatureof 140 F., employing the aluminum strip itself as the cathode and a pairof lead sheets as the anodes. Thence the aluminum strip was passed,after first cooling by means of acid sprays as shown in FIG. 1, into thebaffle section. In the baffle section the average current density rangewas 50 to 80 amperes per square foot, and the average voltage was 13 to15 volts. The time of exposure was about 4 seconds. The strip then waspassed into the main anodizing tank equipped with 11 successively placedpairs of spaced and interconnected lead cathode plates, each pair beingabout 6 inches wide, the length increasing progressively as follows: 11,11, 12, 13, 14, 14, 15, 16,19, 24, and 31 inches, respectively.Conditions prevailing at the first and last pairs of cathode for eachrun were as follows:

FIRST CATHODE PAIR After emerging from the main anodizing section thestrip was rinsed with Water and steam sealed in accordance withconventional procedures. The resulting coating is uniform in apperance,free from voids and defects and burns. Film thickness range is about0.20 to 0.30 mil. The anodized strip is capable of being dyed withorganic dyes to attractive shades.

While we have illustrated and described present preferred embodiments ofthe invention, it will be recognized that the invention may be otherwisevariously embodied and practiced within the scope of the followingclaims.

We claim:

1. In the art of anodizing aluminum, the method comprising the steps of:

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of an aluminum article by immersing said article in anacidic dissolving electrolyte,

applying an electric current to the aluminum 'as anode at an averageanodizing cur-rent density not in excess of amperes per sq. ft.,employing an initial current density in the range of 20 to 600 amperesper sq. ft. and controlling the anodizing rate to prevent burning,

completing the formation of said resistive layer during a period notexceeding about 60 seconds; and

(b) continuing the anodizing of said article at increased anodizingcurrent density to form the main body of the final anodized layerthereon, the thickness of said main body being 'at least about 50 timesthat of said resistive layer, by

passing an electric cur-rent through an acidic dissolving electrolytc,between a cathode immersed therein and said article as anode, 'at anaverage anodizing current density much higher than 100 amperes per sq.ft.

2. In the art of anodizing aluminum, the method comprising the steps of:

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of an aluminum article by immersing said article in anacidic dissolving electrolyte,

applying an electric current to the aluminum as anode at an averageanodizing current density not in excess of 100 amperes per sq. ft.,employing an initial current density in the range of 20 to 600 amperesper sq. ft. and controlling the anodizing rate to prevent burning,

completing the formation of said resistive layer during a period notexceeding about 60 seconds; and

(b) continuing the anodizing of said article at increased anodizingcurrent density to form the main body of the final anodized layerthereon, the thickess of said main body being about 50 to about 200times that of said resistive layer, by

passing an electric current through an acidic dissolving electrolyte,between a cathode immersed therein and said article as anode, at anaverage anodizing current density much higher than 100 amperes per sq.ft., progressively increasing the anodizing current density to a valueabove 250 amperes per sq. ft.

3. In the art of anodizing aluminum, the method comprising the steps of:

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of an aluminum article by immersing said article in anacidic dissolving electrolyte comprising aqueous sulfuric acid solution,applying an electric current to the aluminum as anode at an averagecurrent density of about 50 amperes per sq. ft., while controlling theanodizing rate to prevent burning,

11 completing the formation of said resistive layer to a thickness of atleast about 90 to 110 Angstrom units during a period not exceeding 60seconds; and (b) continuing the anodizing of the article in saidelectrolyte at an average anodizing current density much higher than 100amperes per sq. ft. to form the main body of the final anodized layerthereon, the thickness of said main body being at least about 50 timesthat of said resistive layer. 4. In the art of anodizing aluminum, themethod comprising the steps of:

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of an aluminum article by immersing said article in anacidic dissolving electrolyte comprising aqueous sulfuric acid solution,applying an electric current to the aluminum as anode at an averagecurrent density in the range of to 100 amperes per sq. ft. and anaverage anodizing voltage of about 14 volts, while controlling theanodizing rate to prevent burning, completing the formation of saidresistive layer during a period not exceeding 60 seconds; and (b)continuing the anodizing of said article at increased anodizing currentdensity to form the main body of the final anodized layer thereon, thethickness of said main body being at least about 50 times that of saidresistive layer, by

passing an electric current through an acidic dissolving electrolytecomprising aqueous sulfuric acid solution, between a cathode immersedtherein and said article as anode, at an average anodizing currentdensity much higher than 100 amperes per sq. ft., progressivelyincreasing the anodizing current density to a value above 250 amperesper sq. ft. 5. In the art of continuously anodizing aluminum in stripform, the method comprising the steps of;

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of aluminum strip by feeding said strip through an acidicdissolving electrolyte and applying an electric current to the strip asanode at an average anodizing current density not in excess of 100amperes per sq. ft, employing an initial current density in the range of20 to 600 amperes per sq. ft. and controlling the anodizing rate toprevent burna completing the formation of said resistive layer during aperiod not exceeding 60 seconds; then (b) continuing the anodizing ofsaid strip at increased anodizing current density to form the main bodyof the final anodized layer thereon, the thickness of said main bodybeing at least about 50 times that of said resistive layer, by

advancing the strip through an acidic dissolving electrolyte and betweensuccessive pairs of split cathodes therein, passing an electric currentthrough said electrolyte, between said cathodes and the strip as anode,at an average anodizing current density much higher than 100 amperes persq. ft. and with variations in current density along the strip betweenadjacent pairs of cathodes limited to about 6. In the art ofcontinuously anodizing aluminum in strip form, the method comprising thesteps of:

(a) forming a uniform, dense and electrically-resistive layer on thesurface of aluminum strip by feeding said strip through a pretreatmentportion of an acidic dissolving electrolyte, applying an electriccurrent to the aluminum as anode and to the electrolyte as cathode, atan average anodizing current density not in excess of 100 amperes persq. ft, employing an initial current density in the range of 20 to 600amperes per sq. ft. and controlling the anodizing rate to preventburning,

completing the formation of said resistive layer during an initialperiod not exceeding 60 sec onds, then (b) continuing the anodizing ofsaid strip at increased anodizing current density to form the main bodyof the final anodized layer thereon, the thickness of said main bodybeing at least about 50 times that of said resistive layer, by

advancing the strip into a main portion of the electrolyte and betweensuccessive pairs of split cathodes therein,

passing an electric current through the electrolyte, between saidcathodes and the strip as anode, at an average anodizing current densitymuch higher than 100 amperes per sq. ft; and

(c) limiting the flow of current via the electrolyte between therespective portions thereof to control the anodizing rate of the stripin the pretreatment portion of the electrolyte.

7. In the art of anodizing aluminum, the method comprising the steps of:

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of an aluminum article by immersing said article in anacidic dissolving electrolyte,

applying an electric current to the aluminum as anode at an averageanodizing current density not in excses of 100 amperes per sq. ft.,employing an initial current density in the range of to 600 amperes persq. ft. when 1415 volt average potential is first applied, andcontrolling the anodizing rate to prevent burning,

completing the formation of said resistive layer during a period notexceeding about 60 seconds; and

(b) continuing the anodizing of said article at increased anodizingcurrent density to form the main body of the final anodized layerthereon, the thickness of said main body being at least about 50 timesthat of said resistive layer, by

passing an electric current through an acidic dissolving electrolyte,between a cathode immersed therein and said article as anode, at anaverage anodizing current density much higher than amperes per sq. ft.

8. In the art of continuously anodizing aluminum in strip form, themethod comprising the steps of:

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of aluminum strip by feeding said strip through apretreatment portion of an acidic dissolving electrolyte comprisingaqueous sulfuric acid solution and applying an electric current to thestrip as anode at an average anodizing current density not in excess of100 amperes per sq. ft., employing an initial current density in therange of 80 to 600 amperes per sq. ft. when 14-15 volt average potentialis first applied and controlling the anodizing rate to prevent burning,

completing the formation of said resistive layer Spring a period notexceeding about 6 seconds;

(1)) continuing the anodizing of said strip at increased anodizingcurrent density to form the main body of the final anodized layer, thethickness of said main body being at least about 50 times that of saidresistive layer, by

advancing the strip into a main portion of the electrolyte and betweensuccessive pairs of split cathodes therein,

13 passing an electric current through the electrolyte, between saidcathodes and the strip as anode, at an average anodizing current densitymuch higher than 100 amperes per sq. ft. 9. In the art of continuouslyanodizing aluminum in strip form, the method comprising the steps of:

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of an aluminum strip by feeding said strip through apretreatment portion of an acidic dissolving electrolyte comprisingaqueous sulfuric acid solution and applying an electric current to thestrip as anode, at an average current density in the range of 20 to 100amperes per sq. ft. and an average anodizing voltage of about 14 volts,while controlling the anodizing rate to prevent burning, completing theformation of said resistive layer during a period not exceeding about 6seconds; then (b) continuing the anodizing of said strip at increasedanodizing current density to form the main body of the final anodizedlayer thereon, the thickness of said main body being at least about 50times that of said resistive layer, by

advancing the strip into a main portion of the electrolyte and betweensuccessive pairs of split cathodes therein, passing an electric currentthrough the electrolyte, between said cathodes and the strip as anode,at an average anodizing current density much higher than 100 amperes persq. ft. and with variations in current density along the strip betweenadjacent pairs of cathodes limited to about 30%. progressivelyincreasing the anodizing current density to above 250 amperes per sq.ft. 10. In the art of continuously anodizing aluminum in strip form, themethod comprising the steps of:

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of aluminum strip by feeding said strip through apretreatment portion of an acidic dissolving electrolyte comprisingaqueous sulfuric acid solution and applying an electric current to thestrip as anode, at an average anodizing current density of about 50amperes per sq. ft. and an average anodizing voltage of about 14 volts,while controlling the anodizing rate to prevent burning, completing theformation of said resistive layer stirring a period not exceeding about6 seconds;

en (b) continuing the anodizing of said strip at increased anodizingcurrent density to form the main body of the final anodized layerthereon, the thickness of said main body being at least about 50 timesthat of said resistive layer, by

advancing the strip into a main portion of the electrolyte and betweensuccessive pairs of split cathodes therein,

passing an electric current through said electrolyte, between saidcathodes and the strip as anode, at an average anodizing current densitymuch higher than amperes per sq. ft., the variation in voltage betweenadjacent pairs of cathodes not exceeding about 1 volt and correspondingvariations in current density along the strip being limited to about30%. progressively increasing the anodizing current den= sity to a valuein the range from 250 to about 1000 amperes per sq. ft. 11. 'In the artof continuously anodizing aluminum in strip form, the method comprisingthe steps of:

(a) forming a uniform, dense and electrically-resistive oxide layer onthe surface of aluminum strip by feeding said strip through apretreatment portion of an acidic dissolving electrolyte and applying anelectric current to the strip as anode, at an average anodizing currentdensity in the range of 20 to 100 amperes per sq. ft. and an averageanodizing voltage of about 14 volts, while controlling the anodizingrate to prevent burning, completing the formation of said resistivelayer during a period not exceeding about 6 seconds; then (b) continuingthe anodizing of said strip at increased anodizing current density toform the main body of the final anodized layer thereon, the thickness ofsaid main body being at least about 50 times that of said resistivelayer, by

advancing the strip into a main portion of the electrolyte, passing anelectric current through said electrolyte, between a cathode immersedtherein and the strip as anode, at an average anodizing current densitymuch higher than 100 amperes per sq. ft.; and (c) limiting the flow ofcurrent via the electrolyte between the respective portions thereof tocontrol the anodizing rate of the strip in the pretreatment portion ofthe electrolyte.

References Cited in the file of this patent UNITED STATES PATENTS2,098,774 Coursey et a1. Nov. "9, 1937 2,174,840 Robinson et a1. Oct. 3,1939 2,474,181 De Long June 21, 1949 2,538,317 Mason et al Jan. 16, 19512,692,851 Burrows Oct. 26, 1954 2,812,295 Patrick Nov. 5, 1957 2,844,529Cybriwsky et a1 July 22, 1958 2,855,350 Ernst Oct. 7, 1958 2,901,412Mostovych et a1 Aug. 25, 1959 FOREIGN PATENTS 204,544 Australia Nov. 21,1956 467,024 Great Britain June 9, 1937 761,196 Great Britain Nov. 14,1956

1. IN THE ART OF ANIDIZING ALUMINUM, THE METHOD COMPRISING THE STEPS OF:(A) FORMING A UNIFORM, DENSE AND ELECTRICALLY-RESISTIVE OXIDE LAYER ONTHE SURFACE OF AN ALUMINUM ARTICLE BY IMMERSING SAID ARTICLE IN ANACIDIC DISSOLVING ELECTROKYTE, APPLYING AN ELECTRIC CURRENT TO THEALUMINUM AS ANODE AT AN AVERAGE ANODIZING CURRENT DENSITY NOT IN EXCESSOF 100 AMPERES PER SQ.FT., EMPLOYING AN INITIAL CURRENT DENSITY IN THERANGE OF 20 TO 600 AMPERES PER SQ.FT. AND CONTROLLING THE ANODIZING RATETO PREVENT BURNING, COMPLETEING THE FORMATION OF SAID RESISTIVE LAYERDURING A PERIOD NOT EXCEEDING ABOUT 60 SECONDS; AND (B9 CONTINUING THEANODIZING OF SAID ARTICLE AT INCREASED ANODIZING CURRENT DENSITY TO FORMTHE MAIN BODY OF THE FINAL ANODIZED LAYER THEREON, THE THICKNESS OF SAIDMAIN BODY BEING AT LEAST ABOUT 50 TIMES THAT OF SAID RESISTIVE LAYER, BYPASSING AN ELECTRIC CURRENT THROUGH AN ACIDIC DISSOLVING ELECTROLYTE,BETWEEN A CATHODE IMMERSEC THEREIN AND SAID ARTICLES AS ANODE, AT ANAVERAGE ANODIZING CURRENT DENSITY MUCH HIGHER THAN 100 AMPERES PERSQ.FT.